Passenger cars and trucks on U.S. highways wear out tens of millions of tires each year. Disposal of these used tires has become a major environmental problem. A high proportion (up to 25%-35%) of the weight of a used tire consists of carbon black reinforcing of the rubber in both the tread and sidewalls. This carbon black is prepared by conventional carbon black production processes and comprises individual particles one micron or less in diameter. Fifty to sixty percent (50%-60%) of the weight of a discarded tire is butadiene-styrene copolymer rubber. Tires also contain large amounts of oil and significant quantities of steel wire and/or fiberglass cord. All of these components are expensive and require large amounts of energy in their manufacture. A process that would allow economic recovery of these materials from the huge stocks of used tires piling up around the country would be very desirable. Unfortunately, the very characteristics that make tires long-lasting and safe on the road, i.e., durability, resistance to puncture and slicing, and resistance to decomposition at moderate temperatures; combine to make tires exceptionally difficult to recycle.
The prior art teaches that rubber can be pyrolyized in the absence of air at temperatures of between 842.degree. and 1112.degree. Fahrenheit in laboratory equipment to produce oil, gas and a solid residue that is carbonaceous in nature. Large electrically heated sink reactors and Dewar flasks have been used for obtaining test data.
The prior art also teaches some pilot plants that were built to carry tire processing schemes into the commercial world. Circulating heated ceramic balls have been used as a direct source of reaction heat. The balls are heated externally, mixed with rubber feed chips, discharged, screened, reheated and recycled. These reactions take place substantially at atmospheric pressure. Other pilot plants have been designed which make the carbonaceous solid phase of tire pyrolysis into fuel briquets. These fuel briquets are much less valuable than the carbon black produced by the present invention. Still other batch pilot plants have been built in which the tires are indirectly heated through the tray walls of multi-tray reactors to temperatures of between 1400.degree. and 1600.degree. Fahrenheit. At these temperatures, heavy oils and tar products can be recycled for further cracking to improve carbon black yields. Other batch type process plants have been built that depend on indirect heating through the walls of a jacketed screw reactor from a high temperature molten salt heat sink. These reactors do not have hollow shafts nor hollow flights and have far less heat transfer area than the present invention.
It is extremely difficult to physically break tires apart to obtain individual rubber particles free of reinforcing materials. This is particularly true when, as in the prior art, indirectly heated reactors must transfer heat from a heat source to solid tire particles through a wall. Commercially available tire disintegrators include slicing machines, hammer mills, debeaders and manglers that have been adopted to tire reduction from other industries.
The recent introduction of steel reinforcing in both passenger and truck tires has greatly increased the difficulty and expense of sufficiently disintegrating a tire to convert it into a useable pyrolysis feed stock.
Aside from the purely physical problems associated with breaking down used tires before they can be pyrolized, the prior art also teaches that vapors produced from tire pyrolysis are loaded with dusty unburned rubber and carbon black particles. These particles plug up vapor lines, coolers, condensors, and generally gum up equipment. Further, much of the heavier hydrocarbons driven off during pyrolysis is composed of tar and pitch. These high melting point fractions solidify quickly, especially in the presence of dust and again generally gum up downstream processing equipment. If the tires are wet, as happens when they are washed with water to remove accumulated dust and mud, the water vapor distills and forms emulsions with the heavy oils and tars. Tire pyrolysis oils are also contaminated with metals and solids carryover. This causes them to have greatly reduced value as fuel oils. The high degree of metallic impurities also causes significant problems to be associated with the use of these hydrocarbons for fuel.
The present invention overcomes the physical difficulties of the prior art in preparing used tires as a feedstock by first slicing the tires and then disrupting them to free reinforcing materials. The problems of the prior art with tar and heavy hydrocarbons are avoided by taking out a separate tar or high boiling point condensate side stream. Heat transfer efficiency of the present invention is greatly increased by refluxing a portion of this tar or high boiling point condensate through the vapor scrubber into the solid phase of the pyrolysis reaction.
Solid phase pyrolysis reaction products taught by the prior art include partially decomposed rubber, carbon black particles, fiberglass, steel wire, metallic oxide ashes and dust. The prior art has never taught any satisfactory way of converting this conglomerate carbonaceous mixture into a clean fuel. It is even more difficult to convert such a mixture of components into saleable quantities of carbon black, which would yield much greater economic returns. Because of these difficulties and the environmental restraints placed on such recovery processes, the prior art does not teach a pyrolysis system for the conversion of vehicle tires to saleable carbon black and hydrocarbons.
It is an object of the present invention to teach a method of and teach apparatus for pyrolyzing used tires economically into commercial quantities of tar, oil and fuel gas.
It is a further object of this invention to teach a method of recovering steel scrap from used tires.
It is yet another object of the present invention to teach a method of pyrolyzing used tires that is energy efficient and generates all the fuel gas necessary to operate the process within environmental regulations from the process itself.
It is yet another object of the present invention to teach a method of and teach apparatus for pyrolyzing used tires economically into commercial quality and quantities of carbon black.